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Performance of solid electrolyte oxygen sensor with solid and liquid reference electrode for liquid metal
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Performance of solid electrolyte oxygen sensor with solid and liquid reference electrode for liquid metal Pribadi Mumpuni Adhi a,∗ , Masatoshi Kondo b , Minoru Takahashi b a b
Department of Nuclear Engineering, Tokyo Institute of Technology, Japan Laboratory for Advanced Nuclear Energy, Institute of Innovative Research, Tokyo Institute of Technology, Japan
a r t i c l e
i n f o
Article history: Received 21 June 2016 Received in revised form 12 September 2016 Accepted 4 October 2016 Available online xxx Keywords: Solid electrolyte sensor Lead-bismuth eutectic Liquid reference electrode Solid reference electrode Zirconia
a b s t r a c t As the performance of the solid electrolyte oxygen sensor, the effects of an amount of residual air in the reference electrode (RE) on the stabilization time to an equilibrium condition was investigated for solid electrolyte oxygen sensors with solid Fe–Fe3 O4 RE and liquid metal Bi–Bi2 O3 RE in the tests in an air atmosphere with constant oxygen potential. It was found that the reduction of the volume of the gas region in the reference compartment by an inert material make the stabilization time of RE shorter, where the stabilization time is the time required to the changes from initial setup conditions to the equilibrium conditions in the reference electrode sides. Under the equilibrium conditions of the oxygen potentials both in the solid Fe–Fe3 O4 RE and liquid metal Bi–Bi2 O3 RE, the sensor output showed the stable cell potential which agreed well with the theoretical one given by the Nernst equation. It was also found that the stabilization time of solid Fe–Fe3 O4 RE was shorter than that of liquid metal Bi–Bi2 O3 RE in the air atmosphere. The sensors with the solid Fe–Fe3 O4 RE and liquid metal Bi–Bi2 O3 RE were tested in the atmospheres of a static molten lead-bismuth eutectic (LBE) where the oxygen potentials were controlled to be those with the PbO and Fe3 O4 formation potentials by mass-exchanger method. Under the equilibrium conditions of oxygen potential both in the reference electrodes and in the molten LBE atmospheres, the sensor output showed the stable cell potential which agreed well with the theoretical ones given by the Nernst equation. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Solid electrolyte made of stabilized zirconia is the special ceramic material that has been utilized in various applications such as solid oxide fuel cells, oxygen separators, oxygen pumps, and electrochemical gas sensor [1]. Particularly, the electrochemical gas sensors made of stabilized zirconia solid electrolyte have been utilized for the control of oxygen concentration in steel making process [2] and the control in the combustion of the automotive engine [3]. It is also a promising candidate of oxygen sensor for nuclear reactors such [4] as the lead- and lead-bismuth-cooled fast reactors (LFRs) as one of Generation IV nuclear reactor system concepts [5] and the accelerator-driven system (ADS). Adequate control of oxygen potentials in the lead and leadbismuth coolants has been required to suppress the corrosion of structural materials [6]. In order to control the oxygen potential, the oxygen potential in lead-bismuth eutectic (LBE) must be
∗ Corresponding author. E-mail address: [email protected] (P.M. Adhi).
monitored [7]. A zirconia solid electrolyte type oxygen sensor has been developed and tested as a promising candidate of the instruments for oxygen concentration measurement in LFRs [8]. The oxygen potentials in gas, lead, and LBE can be measured by the solid electrolyte sensor in which the oxygen concentration of a reference electrode (RE) is kept constant. There are three kinds of RE materials, i.e., gas, liquid metal, and solid powder with gas. The performance of the RE is important to have reliable sensor output with desired accuracy and fast response for the measurement in LBE. The gas type RE such as air with Pt has been tested in liquid LBE and lead [8–10]. The sensors with the gas type RE are more complicated than those with the solid and liquid type REs because of additional gas supply system in the structure. The liquid type REs have been tested in liquid LBE and lead, where the RE was made of a mixture of oxygen-saturated Bi and enough Bi2 O3 [8–14] and the mixture of oxygen-saturated In and enough In2 O3 [15,16]. The solid type RE made of the mixture of solid metal powder and metal oxide powder containing a continuous gas phase between the solid powder was studied recently [17]. If the oxygen potential in the gas becomes equilibrium with that of metal oxide powder in the solid
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Nomenclature Activity of oxygen Solubility limit of Fe in LBE (wt%) Oxygen concentration in LBE (wt%) Oxygen concentration of oxygen saturated condition in LBE (wt%) E Cell potential (V) F Faraday constant, = 96485 (C/mol) EX Excess molar Gibbs free energy of LBE (J/mol) GO,LBE PO2 ’ Oxygen partial pressure in working electrode side (in air or LBE) (Pa) PO2 ” Oxygen partial pressure in reference side (Pa) R Gas constant, = 8.314 (J/K.mol) Absolute temperature (K) T O Activity coefficient of oxygen G0 Bi2 O3 Gibbs free energy for formation of Bi2 O3 (J/mol) G0 Fe3 O4 Gibbs free energy for formation of Fe3 O4 (J/mol) aO CFe,s CO CO,s
type RE, the sensors could perform well under any conditions of oxygen potential and temperature in lead and LBE. As for the oxygen sensors with solid and liquid type REs, fast sensor stabilization time and good performance are required without losing the function of the internal RE by the oxidation [18]. The stabilization time means the time required for the internal reference material to reach the redox equilibrium condition from initial setup condition. Fast sensor stabilization time is necessary to detect the change of oxygen concentration and suppress the corrosion in LFRs. Therefore, one of the issues for the oxygen sensors with solid and liquid type REs is a sensor stabilization time. Another issue is the effect of the presence of air in the RE compartment on the performance of oxygen sensor through oxidation. In the present study, some series of experiments using solid and liquid type RE was performed in air and LBE atmosphere to investigate the sensor stabilization time and the effect of the presence of air in the RE compartment on the performance of oxygen sensor. The performance of the sensor with Fe–Fe3 O4 solid type RE was compared with liquid type RE using Bi–Bi2 O3 in the air and molten LBE atmosphere. The cell potential was obtained in the experiments, and the experimental cell potentials were compared with the theoretical one derived from the Nernst equation. The equilibrium condition of oxygen sensor system was investigated by modifying the condition of its internal reference and oxygen potential in working electrode. 2. Experimental apparatuses and procedures
surface of the sensor and painted with silver paste (Dotite D550) at the bottom round tip of the sensor. Silver, gold, and platinum, were compared with each other as electrodes for oxygen sensors in the literature [19]. It was reported that silver was a better electrode material in low-temperature oxygen sensor (at 600 ◦ C). At atmospheric pressure of air and temperature higher than 150 ◦ C, the silver oxide would decompose to silver and oxygen by this reaction AgO2 = Ag + O2 [20]. Therefore, silver paste would be stable without oxidation in air at atmospheric pressure and temperature 600 ◦ C. The binary mixtures of metal (M) and its oxide (My Oz ) were chosen as reference electrodes (RE). When the sensor exposes to high temperature the metal will be oxidized and the oxygen partial pressure in RE is determined by the redox equilibrium reaction as follows: 2 2 M + O2 ↔ My Oz y z
(1)
Table 1 shows the initial conditions of the binary mixtures for solid and liquid type RE. The depths of the binary mixtures in the sensor tube were around 20 mm for all the sensors type a, b, and c. A 1 mm-diameter kanthal lead wire was also used on reference side. The initial amount of oxygen inside the sensor compartment tube was estimated from the volumetric fraction of oxygen of 21% in the initial air volume in the sensor compartment, where the initial air volume was given by the volume of admixture powder and known total sensor compartment volume. The volume of admixture powder was calculated from the mass of powder and the density of the powder. The densities of the admixture powders Fe–Fe3 O4 and Bi–Bi2 O3 were 3.406 g/cm3 and 12.9 g/cm3 , respectively, according to the measurement based on Archimedes’ principle. For sensor type b and c, the top part of the sensor sealed with liquid adhesive Aron ceramic (containing Al2 O3 ). Sealing with Al2 O3 is usually porous. However, without cracks, the air ingression rate was considered very low compared with rate of chemical reaction in reference electrode in the condition of thick Al2 O3 sealing of 15 mm. In the case of the sensor type a, the volume of gas region inside the sensor tube was minimalized by filling up the gas space with liquid ceramic adhesive Aremco 516 (containing ZrO2 –ZrSiO4 ). The top of the tube was covered with SS304 cylinder and sealed with another liquid adhesive Aron ceramic with the thickness of 15 mm. These two adhesives were inert material and were solidified at high temperature. These two solid sealing materials at the top and inside sensor compartment could reduce the possibility of oxygen ingression from the outside, even if the sealing made with Al2 O3 are usually porous. The galvanic cells in air atmosphere experiment are expressed by Kanthal, Ag, O2 (21%)|MSZ|Fe3 O4 , Kanthal.
2.1. Structure of solid electrolyte sensor Kanthal, Ag, O2 (21%)|MSZ|Bi2 O3 , Kanthal. Fig. 1 shows the schematic drawings of sensors used for the present tests. Solid electrolyte sensors made of magnesia-partially stabilized zirconia with 9% mol of Mg (MSZ, ZR–9 M) from Nikkato Company Japan were used. Both MSZ and yttria stabilized zirconia (YSZ) material as solid electrolyte practically can be used in molten LBE. However, MSZ gave potentials in better agreement with theoretically calculated ones than YSZ [16]. In addition, MSZ is also suitable for molten metal according to the manufacturer. Therefore, we chose the magnesia-stabilized zirconia in the present study. The inner and outer diameters of the sensors were 7 mm and 10 mm, respectively, and the lengths of the sensors were 50 mm for sensors type a and b, and 100 mm for sensor type c as shown in Fig. 1(a). For sensors used in an air atmosphere, a 1 mm-diameter kanthal lead wire as working electrode (WE) was wound on the outer
The cell potential at equilibrium condition E is expressed by Nernst equation:. E(V) =
PO ” RT ln 2 4F PO2 ’
(2)
where F is the Faraday constant, T is the absolute temperature of the sensor, R is the gas constant, PO2 ” and PO2 ’ denotes the oxygen partial pressure in reference side and working electrode side (in air), respectively. The oxygen partial pressure in the working electrode is assumed to be 20,946 Pa only for the experiment in air atmosphere. Fig. 1(b) shows the schematic drawing of the oxygen sensor used in LBE atmosphere. Solid electrolyte sensors made of MSZ were
Please cite this article in press as: P.M. Adhi, et al., Performance of solid electrolyte oxygen sensor with solid and liquid reference electrode for liquid metal, Sens. Actuators B: Chem. (2016), http://dx.doi.org/10.1016/j.snb.2016.10.003
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Fig. 1. Schematic drawings of sensors, (a) solid metal and metal oxide RE for testing in an air atmosphere, (b) solid metal and metal oxide RE for testing in LBE atmosphere.
Table 1 Binary mixtures for reference electrode (RE). Reference RE type
Powder
Size
Purity (%)
Mass ratio of metal to oxide (−)
Test in air atmosphere
Test in LBE atmosphere
Solid
Fe Fe3 O4 Bi Bi2 O3
325 mesh – 200 mesh <22 mesh
99 95 99.99 99.999
4:1
1.060
0.625
19:1
2.028
1.030
Liquid
used. They were 5 mm in inner diameter, 8 mm in outer diameter and 50 mm in length. Two types of RE were used: Fe–Fe3 O4 and Bi–Bi2 O3 . The total amount of Fe–Fe3 O4 and Bi–Bi2 O3 powders are shown in Table 1. Mo wires with 1 mm of diameter were used as lead wires in RE and WE of LBE. The sensor compartment was filled with liquid ceramic Aremco 516 and sealed at the top by Aron ceramic. The galvanic cells for testing in LBE atmosphere are expressed by
The oxygen partial pressure in oxygen saturated condition in LBE can be expressed [21] by
PO2 ,s
0.5
PO
0.5
2
The oxygen partial pressure in the solid type Fe − Fe3 O4 RE PO2 ”, is calculated by G0 Fe3 O4
(3)
4
The oxygen activity aO in LBE can be expressed by the oxygen concentration in LBE CO and the oxygen partial pressure in LBE PO2 ’ [12] as aO = O CO =
CO = CO,s
PO2 PO2 , s
0.5 (4)
where O is activity coefficient, and the subscript ‘s’ indicates the saturation condition in LBE. Eq. (4) can be re-arranged to Eq. (5).
PO2
0.5
= O CO PO2 ,s
0.5
EX GO,LBE
(6)
RT
= CO exp
EX GO,LBE
(7)
RT
EX The value of GO,LBE relative to 1 wt% standard state is given by [22]
Mo, LBE|MSZ|Bi-Bi2 O3 , Mo
RT ln PO2 ” = 2
= CO,s exp
EX where GO,LBE is excess mollar Gibbs free energy of dissolution of oxygen. The oxygen partial pressure in LBE can be obtained by substituting Eq. (6) for Eq. (5)
Mo, LBE|MSZ|Fe-Fe3 O4 , Mo
Total mass (g)
(5)
EX GO,LBE (J/mol) = -127398+49.272T
(8)
By substituting Eqs. (3) and (7) for Eq. (2), the cell potential at equilibrium condition E is expressed by the oxygen concentration in Fe − Fe3 O4 RE CO as E=
RT 2F
G0 Fe3 O4 4RT
EX GO,LBE
−
RT
− ln CO
,
(9)
As the oxygen partial pressure in Bi–Bi2 O3 RE, PO2 ” is given by G0 Bi2 O3 RT ln PO2 ” = 2 3
(10)
the cell potential at equilibrium condition E is expressed by the oxygen concentration in Bi–Bi2 O3 RE, CO as RT E= 2F
G0 Bi2 O3 3RT
−
EX GO,LBE
RT
− ln CO
,
(11)
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4 Table 2 Types of sensors and experimental conditions. Atmosphere
Reference electrode
Experiment No.
Sensor Type
Final Temperature (◦ C)
Volume of O2 in sensor compartment (mL)a
Mole number of O2 in sensor compartment (mol)a
Air
Fe–Fe3 O4
1 2 3 4 5 6 7
a b c a b d d
600 600 600 600 600 550 550
0.103 0.588 1.400 0.135 0.620 0.044 0.066
4.21 × 10−6 2.40 × 10−5 5.70 × 10−5 5.53 × 10−6 2.53 × 10−5 1.81 × 10−6 2.71 × 10−6
Bi–Bi2 O3 LBE a
Fe–Fe3 O4 Bi–Bi2 O3
Estimated in atmosphere at 25 ◦ C.
In case of oxygen-saturated LBE, CO is equal to the solubility of oxygen CO,S , and expressed by the Schroer correlation [23] as logCO,s (wt%) = 2.2 − 4416/T (T ≤ 1023K)
(12)
2.2. Experimental conditions 2.2.1. Performance in air atmosphere Fig. 2(a) shows the schematic drawing of experimental apparatus used for the test in an air atmosphere with constant oxygen concentra tion of 21%. The sensor was put in the stainless steel vessel. The distance between the sensor tip and vessel surface was 1 cm. The air temperature around the sensor was kept uniform and constant by filling a heat insulator inside a stainless steel vessel. The sensor temperature was measured with the sheathed thermocouple wound on the outer surface of the sensor. Table 2 shows the sensor types and experimental conditions No.1–No.5 in the air atmosphere. Three different types of sensor (a, b, and c) were tested, and the final temperature of this experiment was 600 ◦ C. The cell potential depends on the condition on the reference electrode side. The cell potential was measured using the electrometer Model EM-05 (Toho Technical Research Co., Ltd.) that was specially ordered based on our requirement of specifications for the cell potential measurement, in particular, the input impedance. The input impedance of the electrometer is >1012 G. 2.2.2. Performance in molten LBE atmosphere Fig. 2(b) shows the schematic drawing of experimental apparatus used in the test for the sensors with Fe–Fe3 O4 and Bi–Bi2 O3 REs in molten LBE atmosphere, where the amount of LBE was 450 g. The sensor tube, electrical lead wire, and sheathed thermocouple were inserted in the LBE in the closed cylindrical SS304 crucible within 50 mm in diameter and length. The crucible was evacuated by a vacuum pump and filled with Ar gas with purity 99.99% as a cover gas five times to ensure no air remain inside the crucible. Then, the crucible was heated up, and the cell potential was measured from melting point of LBE to the final temperature of 550◦ C for each sensor. The cell potential was measured using the high impedance electrometer same as previous. Table 2 shows the sensor type and Experiment No.6–No.7 conditions in the LBE atmosphere. 2.3. Control of oxygen potential in molten LBE atmosphere To have a significance difference with the potential in RE side, the oxygen sensor with solid Fe–Fe3 O4 RE was tested in oxygen-saturated LBE and the liquid Bi–Bi2 O3 RE was tested in LBE equilibrium with iron oxide formation. If the sensor with the FeFe3 O4 RE was tested in liquid LBE with Fe3 O4 formation potential, the potential difference of the sensor output signal would be nearly zero voltage. Zero voltage was not good for the observation, since if the signal was shown zero voltage there was the possibility of short circuit. We needed a drastic potential difference between RE
and LBE to see the effect of controlling the oxygen potential by mass-exchanger method clearly. This method to control the oxygen potential is called “mass-exchanger method” that has been studied in static LBE condition [8], and in flow condition in the LBE circulation loop [11]. A numerical model of this method has also been developed [24]. For the test using solid Fe–Fe3 O4 RE, 4 g of PbO powder that was enough according to Eq. (12) was mixed near the surface of the LBE melt to have oxygen saturation condition. Thermodynamic equilibrium is attained through oxygen mass transfer between PbO and the liquid LBE. The chemical reaction of solid PbO, dissolved oxygen [O], and liquid Pb in LBE can be written as PbO(s) ↔ [O]LBE + Pb(l)
(13)
For the test of the oxygen sensor with the liquid metal type RE of Bi–Bi2 O3 , the oxygen potential in LBE was set to be equilibrium with iron oxide formation condition. In order to mix the Fe and Fe3 O4 powder with LBE well even in a static condition, three layers of 150 g LBE at the bottom, mixture of 15 g of Fe powder and 4 g of Fe3 O4 powder layer at the middle, and 300 g of LBE at the top were prepared in the crucible, and melted. Some Fe was necessarily present to be dissolved in LBE and reacted with dissolved oxygen, and the Fe3 O4 was used to keep the equilibrium condition. The reaction of the dissolved iron [Fe], Fe3 O4 and dissolved oxygen [O] in LBE can be written as 3[Fe]LBE + 4[O]LBE ↔ Fe3 O4(s)
(14)
Fe/Fe3 O4 powder should remain as solid phases under the equilibrium condition without dissolving and oxidizing all the Fe into LBE and without reducing all the Fe3 O4 into Fe. It was confirmed that the amount of Fe powder put in LBE was enough according to the Fe solubility limit in LBE [25]: log C Fe,s (wt%) = 2.01 − 4380/T (823 ≤ T ≤ 1053 K)
(15)
The oxygen concentration corresponding to the oxygen potential for the formation of Fe3 O4 in LBE is given by [24]: logCO,min (wt%) = 3.04 −
3 11047 − log CFe,s T 4
(16)
In order to keep this oxygen concentration, Fe3 O4 powder was put enough in LBE taking into account the equilibrium condition in Eqs. (14) and (16). In the case of Fe3 O4 and Fe existing condition, the oxygen potential in LBE is correspondent to that of the F3 O4 formation energy. Therefore, the theoretical cell potential (E) for Bi − Bi2 O3 RE can be written as E=
RT 2F
G0 Bi2 O3 3RT
−
G0 Fe3 O4 4RT
,
(17)
Metallic impurities soluble from the SS304 crucible into LBE might affect the oxygen chemistry in molten LBE. However, the effect of the impurities such as Ni on cell potential was assumed to
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Fig. 2. Schematic drawings of experimental apparatuses for the experiment: (a) air atmosphere and (b) molten LBE atmosphere.
be negligible in this study. From thermochemical-dynamics point of view, nickel may be oxidized in case of controlling using PbO powder, and nickel may be not oxidized in case controlling using Fe/Fe3 O4 powder. In both cases, the amount of PbO or Fe/Fe3 O4 powders was large enough to control the oxygen potential properly. The nickel effect of sensor performance is unknown at present. Therefore, the equilibrium oxygen potential in molten LBE was determined by the presence of PbO or Fe/Fe3 O4 powders.
3. Results and discussion 3.1. Performance of sensors in air atmosphere with constant oxygen potential 3.1.1. Solid Fe − Fe3 O4 reference electrode The working temperature takes an important role in this kind of experiment. According to [1], the threshold temperature is around 360 ◦ C, that is, the temperature that the Nernstian behavior still can be achieved. However, one of the authors found that the threshold temperature was around 450 ◦ C in the same kind of experimental result for solid electrolyte oxygen sensor. The good agreement between measured and theoretical cell potentials was obtained above around 450 ◦ C [26]. Therefore, only the experimental data and theoretical cell potential above the threshold temperature of 450◦ C were included in the later results and discussions. Fig. 3 shows the results of the cell potential in the experiments No 1–No. 3 where the sensors with solid Fe–Fe3 O4 RE was placed in the air atmosphere. The experiment No. 1 that used the sensor type a reached the formation potential of Fe3 O4 faster than the other sensors. The stabilization time of the sensor in experiment No. 1 was the shortest among them. The measured cell potential already agreed with theoretical one after the temperature of the sensor reached 450 ◦ C. The measured cell potentials in the experiments No. 2 and No. 3 were initially higher than the theoretical cell potential at the temperature 450 ◦ C. At 450 ◦ C, the presence of air inside the sensor compartment was still more dominant on cell potential than the formation potential
of Fe3 O4 in experiment No. 2 and No. 3. Therefore, the oxygen partial pressure on the reference side was equal to that of air. The mechanism of the electrode reactions on both surfaces of the solid electrolyte is illustrated in Fig. 4(a). On the solid reference side, the Fe–Fe3 O4 powder just acted like sensing electrode (SE) before the residual oxygen was consumed. Electrochemical reaction of oxygen takes place on the triple-phase boundary (TPB) between Fe metalgas-electrolyte surfaces as an electrode reaction. The reaction is given by: O2(g) + 4e− → 2O2−
(18)
This reaction changes the potential of the SE. The same phenomenon occurs on the working electrode (WE) side, where Ag paste acts as SE When temperature increased, the residual oxygen in the air was consumed gradually by the reaction: 3Fe(s) + 2O2(g) → Fe3 O4(s)
(19)
where the amount of Fe powder inside the sensor compartment was enough for the consumption. As a result, the oxygen partial pressure decreased, and finally the formation and dissociation of Fe3 O4 expressed by Fe3 O4(s) ↔ 3Fe(s) + 2O2(g)
(20)
determined the equilibrium oxygen partial pressure on the reference side The trends of the decrease of measured cell potential in the experiment No. 2 and No. 3 were quite similar to each other after the temperature reached near 600 ◦ C, which suggests that the residual oxygen was consumed in RE in both experiments. The stabilization time at 600 ◦ C of these two sensors was around 10 min. These results gave the evidence that residual air in sensor compartment tube had an effect on the equilibrium condition on the solid Fe–Fe3 O4 RE side. 3.1.2. Liquid Bi–Bi2 O3 reference electrode Fig. 5 shows the results of the cell potential in the experiments No. 4 and No. 5 where the sensors with liquid Bi–Bi2 O3 RE was
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Fig. 3. Cell potential (E) of the sensors in an air atmosphere using solid Fe–Fe3 O4 RE (Experiment No. 1–No. 3). Green, blue, and red colors indicate the Experiment No. 1, 2, and 3, respectively as shown in Table 2. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
placed in the air atmosphere. After the working temperature had reached the threshold temperature of 450 ◦ C, the cell potential (E) of the both of the sensors showed discrepancy with the theoretical E. At 600 ◦ C, the results show that the measured cell potential gradually changed to theoretical value after several hours. However, the stabilization time of the sensor type b in the experiment No. 5 was longer than that of sensor type a in experiment No. 4. The cell potential of the experiment No. 4 during the stabilization time was lower than the theoretical value. The possible explanation for this phenomenon is that the oxygen partial pressure in reference side for experiment No. 4 was initially low since the air was removed by filling with liquid ceramic Aremco 516 (containing ZrO2 –ZrSiO4 ) in sensor compartment. This inert material is used not only to reduce air or gas gap but also to prevent the ingression of air from the outside. Therefore, the cell potential of the experiment No. 4 during the stabilization time was lower than the theoretical value. On the other hand, the experiments No. 2, No. 3, and No. 5 showed higher measured potential than a theoretical one initially because of the presence of air.
The mechanism of the electrode reaction of these two sensors using liquid Bi–Bi2 O3 RE is illustrated in Fig. 4 (b). At RE side, the wettability of electrolyte surface with liquid Bi might poor. Therefore, the gas gap might be formed between electrolyte surface and liquid Bi on the interface, and the reaction can take place either at the double-phase boundary (DPB) of electrolyte and liquid Bi or the triple-phase boundary (TPB) of electrolyte, gas and liquid Bi [27]. When oxygen saturated condition was achieved on the RE side, the O atoms either can be formed from dissociation of O2 molecules or Bi2 O3 . In case of the DPB, the oxygen ions O2− is formed and diffused in the solid electrolyte, and the electrode reaction in liquid RE can be expressed as: 2O ↔ 2O2− + 4e−
(21)
Different behaviors were observed in both of the experiments No. 4 and No. 5, which gave the evidence that air also influenced the cell potential not only for solid type RE but also for liquid type RE through the gas in the gap. The stabilization time of the sensor in the experiment No. 4 is shorter than in the experiment No. 5.
Fig. 4. Electrode reaction models for the oxygen sensors with (a) solid Fe–Fe3 O4 , (b) liquid Bi–Bi2 O3 in air atmosphere.
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Fig. 5. Cell potential (E) of sensors with liquid Bi–Bi2 O3 RE in an air atmosphere (Experiment No. 4–No 5). Blue and red colors indicate the experiments No. 4 and 5 in Table 2, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
It took around 2 h to reach an equilibrium condition for the sensor type a in the experiment No. 4. For the sensor type b in the experiment No. 5, it took around 2.5 h to reach an equilibrium condition. The excess oxygen in the gas region of sensor compartment would remain some time before it diffused and fully consumed by the oxidation of Bi since the liquid Bi was already in oxygen saturated condition. The liquid type RE needed a longer time to reach an equilibrium condition compare to the solid type RE. The oxygen saturated liquid Bi RE could not achieve the equilibrium condition within a short period probably because the effective area of gas phase for the equilibrium reaction of the liquid reference was smaller than that of the solid powder reference. The results of two different volumes for Bi-Bi2 O3 reference electrode in air showed a significant difference of stabilization time. The difference was around 30 min. We could clarify the effect from the two-volume-experiments. On the other hand, for the Fe-Fe3 O4 reference electrode, the difference of stabilization time for each experiment was shorter, i. e., only several minutes. Therefore, it was needed to test at least three different volume for the Fe-Fe3 O4 reference electrode, but it was not for the Bi-Bi2 O3 reference electrode. After the equilibrium condition had been achieved in the liquid RE, the potential of RE was time independent at the constant temperature. In the case of no leakage, the air will not ingress from outside, and the equilibrium condition will remain. The volume of the gas region inside the sensor compartment should be minimalized to have an improvement of design and performance of the sensor. One of the ways is to fill the sensor compartment with inert material as demonstrated above.
3.2. Performance of sensors in molten LBE atmosphere under change in oxygen potential to equilibrium 3.2.1. Solid Fe–Fe3 O4 reference electrode Fig. 6 shows the results of the cell potential in the experiment No. 6 where the sensor with solid Fe –Fe3 O4 RE was placed in molten LBE atmosphere where PbO gradually saturated the oxygen potential. After the LBE had melted in the atmosphere, the achievement to overall equilibrium condition in oxygen sensor system was influenced by the condition of the LBE rather than that condition of RE in the sensor compartment. On RE side, the stabilization time was short, and the equilibrium condition with Fe3 O4 formation potential could be achieved within several minutes as mentioned in
Fig. 6. Cell potential (E) of sensors with solid Fe–Fe3 O4 RE in molten LBE atmosphere where oxygen potential changed to be in equilibrium with PbO formation potential (Experiment No. 6).
Section 3.1.1. Therefore, the dominant factor for the achievement to the overall equilibrium was the change of the condition of working electrode (WE) side of LBE. When the working temperature reached the threshold temperature of 450 ◦ C, the cell potential value was high at first because of low oxygen concentration in LBE, and the cell potential decreased gradually with an increase in oxygen concentration in LBE due to the presence of PbO particle. The mechanism of electrode reaction is illustrated in Fig. 7(a). In LBE, the electrode reaction may take place either at the DPB and TPB. The adsorption-desorption and a partial dissociation of oxygen molecules occur at the DPB. The excess O2 molecules dissolve in LBE and dissociate into O atoms. It was estimated that the PbO powder that dissociated into Pb and O atoms was enough in LBE, and oxygen saturated condition could be achieved. Therefore, the decrease of cell potential observed corresponds to the rise of oxygen concentration in LBE. It was found that the cell potential value was close to PbO formation potential after the temperature reached 470 ◦ C, and finally, the cell potential became constant near the theoretical equilibrium condition. The transient period in LBE from the beginning to reach the equilibrium condition was around 50 min.
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Fig. 7. Electrode reaction models for oxygen sensors with (a) solid Fe–Fe3 O4 , (b) liquid Bi–Bi2 O3 in molten LBE atmosphere.
as oxygen supplier to keep the oxygen concentration in equilibrium with Fe3 O4 formation potential after the amount of oxygen was reduced in the LBE. Slow diffusion of oxygen and Fe in LBE in a static condition caused a relatively long transient period to reach an equilibrium condition. It was reported that the diffusion coefficient of oxygen and Fe in LBE at 550◦ C are 6.3 × 10−6 cm2 /s [21] and 7.6 × 10−6 cm2 /s [28], respectively. In the present result, the total transient period to reached LBE equilibrium with Fe3 O4 formation potential was around 16 h. During this time, the measured cell potential gradually changed until it reached around 0.495 V and became constant at the theoretical equilibrium condition.
Fig. 8. Cell potential (E) of the sensors with liquid Bi–Bi2 O3 RE in molten LBE atmosphere where oxygen potential changed to be in equilibrium with Fe3 O4 formation potential (Experiment No. 7).
3.2.2. Liquid Bi − Bi2 O3 reference electrode Fig. 8 shows the results of the cell potential in the experiment No. 7 where the sensor with liquid Bi–Bi2 O3 RE was placed in molten LBE atmosphere where the oxygen potential gradually changed to equilibrium with Fe3 O4 formation potential. The measured cell potential increased steeply to 0.39 V at first after the working temperature reached 550 ◦ C. However, the equilibrium condition was not attained yet on RE and WE side of LBE. According to the previous result in Section 3.1.2, the stabilization time of RE was around 2 h. Therefore, the dominant factor that influences the sensor output cell potential was the condition of WE side of LBE. The mechanism of electrode reaction is illustrated in Fig. 7(b). It was assumed that the gas gap might also form on the outer surface of the solid electrolyte on the WE side. Therefore, the reaction could take place either at DPB or TPB. The excess oxygen molecules O2 are dissociated in the molten LBE and become oxygen atoms O. The light Fe/Fe3 O4 powder floats on the surface of the LBE due to buoyancy force. The Fe as an impurity diffuses slowly downward in the molten LBE, oxidized by O atoms, and then the product of magnetite Fe3 O4 floats by buoyancy force. At the same time, the oxygen dissolved in the molten LBE diffused upward to the surface and oxidized Fe to form Fe3 O4 . The increase of measured cell potential corresponds to the decrease of oxygen concentration in LBE through these processes. Adding Fe3 O4 powder was required
4. Conclusions The major conclusions of this study are as follows: (1) As the performance of the solid electrolyte oxygen sensor, the effects of an amount of residual air in the reference electrode (RE) on the stabilization time from initial condition to an equilibrium condition was investigated. It was demonstrated that reducing the gas volume in the reference compartment with inert material shortened the stabilization time of reference electrode material to reach an equilibrium condition. (2) The oxygen sensor with the solid Fe–Fe3 O4 reference electrode (RE) was tested in air atmosphere with constant oxygen potential. The cell potential (E) output agreed well with the theoretical one given by Nernst equation after the redox equilibrium condition was achieved in the reference compartment at 600 ◦ C. The stabilization time to the equilibrium in the reference electrode was influenced by the residual air in the sensor compartment. It could be shortened to less than 10 min by decreasing the volume of the gas region in the reference compartment filled with inert material. (3) The oxygen sensor with the liquid Bi–Bi2 O3 RE was tested in air atmosphere with constant oxygen potential. The cell potential agreed well with the theoretical one given by Nernst equation after the redox equilibrium condition was achieved in the reference compartment at 600 ◦ C. The stabilization time to the equilibrium in the reference electrode was influenced by the residual air in the sensor compartment and was longer than that of the sensor with the solid Fe–Fe3 O4 RE even if the volume of residual air was decreased
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by filling the reference compartment with an inert material. The stabilization time was as long as 2 h possibly because the oxygen saturated liquid Bi reference could not achieve the equilibrium condition within a short time. (4) The oxygen sensor with the solid Fe–Fe3 O4 RE that had the shortest stabilization time in the air atmosphere was tested in a static molten LBE atmosphere where the oxygen potential was controlled by PbO. The cell potential agreed with the theoretical one given by the Nernst equation after the oxygen potential in the static LBE reached the equilibrium conditions with the formation potentials of PbO. Since the oxygen potential in the solid Fe–Fe3 O4 RE reached the equilibrium condition faster than in the static LBE. The total transient period to reached LBE equilibrium with PbO formation potential was around 50 min. (5) The oxygen sensor with the liquid Bi–Bi2 O3 RE that had the shortest stabilization time in the air atmosphere was tested in a static molten LBE atmosphere where the oxygen potential was controlled by Fe and Fe3 O4 . The cell potential agreed with the theoretical one given by the Nernst equation after the oxygen potential in the static LBE reached the equilibrium conditions with the formation potentials of Fe3 O4 . Since the oxygen potential in the liquid Bi–Bi2 O3 RE reached the equilibrium condition faster than in the static LBE. The total transient period to reached LBE equilibrium with Fe3 O4 formation potential was around 16 h. References [1] S. Zhuikov, Electrochemistry of Zirconia Gas Sensor, CRC Press, Boca Raton, Florida, 2008. [2] J.W. Fergus, Using chemical sensors to control molten metal processing, J. Miner. Met. Mater. Soc. 52 (2000) http://www.tms.org/pubs/journals/jom/ 0010/fergus/fergus-0010.html. [3] A.M. Azad, S.A. Akbar, S.G. Mhaisalkar, L.D. Birkefeld, K.S. Goto, Solid-State gas sensors: a review, J. Electrochem. Soc. 139 (1992) 3690–3704, http://dx.doi. org/10.1149/1.2069145. [4] J. Zhang, N. Li, Review of the studies on fundamental issues in LBE corrosion, J. Nucl. Mater. 373 (2008) 351–377, http://dx.doi.org/10.1016/j.jnucmat.2007. 06.019. [5] L. Cinotti, C.F. Smith, H. Sekimoto, L. Mansani, M. Reale, J.J. Sienicki, Lead-cooled system design and challenges in the frame of Generation IV International Forum, J. Nucl. Mater. 415 (2011) 245–253, http://dx.doi.org/10. 1016/j.jnucmat.2011.04.042. [6] J. Zhang, Oxygen control technology in applications of liquid lead and lead-bismuth systems for mitigating materials corrosion, J. Appl. Electrochem. 43 (2013) 755–771, http://dx.doi.org/10.1007/s10800-013-0568-8. [7] L. Brissonneau, F. Beauchamp, O. Morier, C. Schroer, J. Konys, A. Kobzova, et al., Oxygen control systems and impurity purification in LBE: learning from DEMETRA project, J. Nucl. Mater. 415 (2011) 348–360, http://dx.doi.org/10. 1016/j.jnucmat.2011.04.040. [8] C. Schroer, J. Konys, A. Verdaguer, J. Abellà, A. Gessi, A. Kobzova, et al., Design and testing of electrochemical oxygen sensors for service in liquid lead alloys, J. Nucl. Mater. 415 (2011) 338–347, http://dx.doi.org/10.1016/j.jnucmat.2011. 04.045. [9] J. Konys, H. Muscher, Z. Voß, O. Wedemeyer, Development of oxygen meters for the use in lead–bismuth, J. Nucl. Mater. 296 (2001) 289–294. [10] J. Konys, H. Muscher, Z. Voß, O. Wedemeyer, Oxygen measurements in stagnant lead–bismuth eutectic using electrochemical sensors, J. Nucl. Mater. 335 (2004) 249–253, http://dx.doi.org/10.1016/j.jnucmat.2004.07.018. [11] M. Kondo, M. Takahashi, Study on control of oxygen concentration in lead–bismuth flow using lead oxide particles, J. Nucl. Mater. 357 (2006) 97–104, http://dx.doi.org/10.1016/j.jnucmat.2006.05.051. [12] G. Muller, G. Schumacher, F. Zimmermann, Investigation on oxygen controlled liquid lead corrosion of surface treated steels, J. Nucl. Mater. 278 (2000) 85–95. [13] A.K. Rivai, T. Kumagai, M. Takahashi, Performance of oxygen sensor in lead-bismuth at high temperature, Prog. Nucl. Energy. 50 (2008) 575–581, http://dx.doi.org/10.1016/j.pnucene.2007.11.043. [14] A.K. Rivai, M. Takahashi, Investigations of a zirconia solid electrolyte oxygen sensor in liquid lead, J. Nucl. Mater. 398 (2010) 160–164, http://dx.doi.org/10. 1016/j.jnucmat.2009.10.027.
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Biographies Pribadi Mumpuni Adhi graduated from Physics Department at Institut Teknologi Bandung (ITB), Indonesia, in 2012 and obtained Master of Eng. in Nuclear Engineering at Tokyo Institute of Technology (Tokyo Tech) in 2015. Currently, he is a Doctoral candidate at Department of Nuclear Engineering, Tokyo Tech. His research focuses on developing oxygen sensor in liquid lead-bismuth eutectic. Masatoshi Kondo obtained his Doctor of Eng. Degree in Nuclear Engineering from Tokyo Tech in 2006. Then, he worked for Nuclear Institute for Fusion Science (NIFS) as an Assistant Professor in 2006. Since 2015, he is an Assistant Professor at Tokyo Institute of Technology (Tokyo Tech). His research focuses on liquid metals and fusion reactor technology. Minoru Takahashi has been a Professor of nuclear engineering at Tokyo Institute of Technology (Tokyo Tech) since 2013. He obtained his Master of Eng. degree and Doctor of Eng. degree in Nuclear Engineering from Tokyo Tech in 1977 and in 1981, respectively. He worked for the development of fast reactors as a researcher at Power Reactor and Nuclear Fuel Development Corporation from 1980 till 1984. Then, he was an Assistant Professor from 1984 till 1997 and an Associate Professor from 1997 till 2013 at Research Laboratory for Nuclear Reactors, Tokyo Tech. His research focuses on thermal-hydraulics and materials for light water reactors, liquid-metalcooled fast reactors, a lead-bismuth-cooled accelerator-driven system and a lithiumcooled fusion reactor blanket.
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Performance of solid electrolyte oxygen sensor with solid and liquid reference electrode for liquid metal Pribadi Mumpuni Adhi a,∗ , Masatoshi Kondo b , Minoru Takahashi b a b
Department of Nuclear Engineering, Tokyo Institute of Technology, Japan Laboratory for Advanced Nuclear Energy, Institute of Innovative Research, Tokyo Institute of Technology, Japan
a r t i c l e
i n f o
Article history: Received 21 June 2016 Received in revised form 12 September 2016 Accepted 4 October 2016 Available online xxx Keywords: Solid electrolyte sensor Lead-bismuth eutectic Liquid reference electrode Solid reference electrode Zirconia
a b s t r a c t As the performance of the solid electrolyte oxygen sensor, the effects of an amount of residual air in the reference electrode (RE) on the stabilization time to an equilibrium condition was investigated for solid electrolyte oxygen sensors with solid Fe–Fe3 O4 RE and liquid metal Bi–Bi2 O3 RE in the tests in an air atmosphere with constant oxygen potential. It was found that the reduction of the volume of the gas region in the reference compartment by an inert material make the stabilization time of RE shorter, where the stabilization time is the time required to the changes from initial setup conditions to the equilibrium conditions in the reference electrode sides. Under the equilibrium conditions of the oxygen potentials both in the solid Fe–Fe3 O4 RE and liquid metal Bi–Bi2 O3 RE, the sensor output showed the stable cell potential which agreed well with the theoretical one given by the Nernst equation. It was also found that the stabilization time of solid Fe–Fe3 O4 RE was shorter than that of liquid metal Bi–Bi2 O3 RE in the air atmosphere. The sensors with the solid Fe–Fe3 O4 RE and liquid metal Bi–Bi2 O3 RE were tested in the atmospheres of a static molten lead-bismuth eutectic (LBE) where the oxygen potentials were controlled to be those with the PbO and Fe3 O4 formation potentials by mass-exchanger method. Under the equilibrium conditions of oxygen potential both in the reference electrodes and in the molten LBE atmospheres, the sensor output showed the stable cell potential which agreed well with the theoretical ones given by the Nernst equation. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Solid electrolyte made of stabilized zirconia is the special ceramic material that has been utilized in various applications such as solid oxide fuel cells, oxygen separators, oxygen pumps, and electrochemical gas sensor [1]. Particularly, the electrochemical gas sensors made of stabilized zirconia solid electrolyte have been utilized for the control of oxygen concentration in steel making process [2] and the control in the combustion of the automotive engine [3]. It is also a promising candidate of oxygen sensor for nuclear reactors such [4] as the lead- and lead-bismuth-cooled fast reactors (LFRs) as one of Generation IV nuclear reactor system concepts [5] and the accelerator-driven system (ADS). Adequate control of oxygen potentials in the lead and leadbismuth coolants has been required to suppress the corrosion of structural materials [6]. In order to control the oxygen potential, the oxygen potential in lead-bismuth eutectic (LBE) must be
∗ Corresponding author. E-mail address: [email protected] (P.M. Adhi).
monitored [7]. A zirconia solid electrolyte type oxygen sensor has been developed and tested as a promising candidate of the instruments for oxygen concentration measurement in LFRs [8]. The oxygen potentials in gas, lead, and LBE can be measured by the solid electrolyte sensor in which the oxygen concentration of a reference electrode (RE) is kept constant. There are three kinds of RE materials, i.e., gas, liquid metal, and solid powder with gas. The performance of the RE is important to have reliable sensor output with desired accuracy and fast response for the measurement in LBE. The gas type RE such as air with Pt has been tested in liquid LBE and lead [8–10]. The sensors with the gas type RE are more complicated than those with the solid and liquid type REs because of additional gas supply system in the structure. The liquid type REs have been tested in liquid LBE and lead, where the RE was made of a mixture of oxygen-saturated Bi and enough Bi2 O3 [8–14] and the mixture of oxygen-saturated In and enough In2 O3 [15,16]. The solid type RE made of the mixture of solid metal powder and metal oxide powder containing a continuous gas phase between the solid powder was studied recently [17]. If the oxygen potential in the gas becomes equilibrium with that of metal oxide powder in the solid
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Nomenclature Activity of oxygen Solubility limit of Fe in LBE (wt%) Oxygen concentration in LBE (wt%) Oxygen concentration of oxygen saturated condition in LBE (wt%) E Cell potential (V) F Faraday constant, = 96485 (C/mol) EX Excess molar Gibbs free energy of LBE (J/mol) GO,LBE PO2 ’ Oxygen partial pressure in working electrode side (in air or LBE) (Pa) PO2 ” Oxygen partial pressure in reference side (Pa) R Gas constant, = 8.314 (J/K.mol) Absolute temperature (K) T O Activity coefficient of oxygen G0 Bi2 O3 Gibbs free energy for formation of Bi2 O3 (J/mol) G0 Fe3 O4 Gibbs free energy for formation of Fe3 O4 (J/mol) aO CFe,s CO CO,s
type RE, the sensors could perform well under any conditions of oxygen potential and temperature in lead and LBE. As for the oxygen sensors with solid and liquid type REs, fast sensor stabilization time and good performance are required without losing the function of the internal RE by the oxidation [18]. The stabilization time means the time required for the internal reference material to reach the redox equilibrium condition from initial setup condition. Fast sensor stabilization time is necessary to detect the change of oxygen concentration and suppress the corrosion in LFRs. Therefore, one of the issues for the oxygen sensors with solid and liquid type REs is a sensor stabilization time. Another issue is the effect of the presence of air in the RE compartment on the performance of oxygen sensor through oxidation. In the present study, some series of experiments using solid and liquid type RE was performed in air and LBE atmosphere to investigate the sensor stabilization time and the effect of the presence of air in the RE compartment on the performance of oxygen sensor. The performance of the sensor with Fe–Fe3 O4 solid type RE was compared with liquid type RE using Bi–Bi2 O3 in the air and molten LBE atmosphere. The cell potential was obtained in the experiments, and the experimental cell potentials were compared with the theoretical one derived from the Nernst equation. The equilibrium condition of oxygen sensor system was investigated by modifying the condition of its internal reference and oxygen potential in working electrode. 2. Experimental apparatuses and procedures
surface of the sensor and painted with silver paste (Dotite D550) at the bottom round tip of the sensor. Silver, gold, and platinum, were compared with each other as electrodes for oxygen sensors in the literature [19]. It was reported that silver was a better electrode material in low-temperature oxygen sensor (at 600 ◦ C). At atmospheric pressure of air and temperature higher than 150 ◦ C, the silver oxide would decompose to silver and oxygen by this reaction AgO2 = Ag + O2 [20]. Therefore, silver paste would be stable without oxidation in air at atmospheric pressure and temperature 600 ◦ C. The binary mixtures of metal (M) and its oxide (My Oz ) were chosen as reference electrodes (RE). When the sensor exposes to high temperature the metal will be oxidized and the oxygen partial pressure in RE is determined by the redox equilibrium reaction as follows: 2 2 M + O2 ↔ My Oz y z
(1)
Table 1 shows the initial conditions of the binary mixtures for solid and liquid type RE. The depths of the binary mixtures in the sensor tube were around 20 mm for all the sensors type a, b, and c. A 1 mm-diameter kanthal lead wire was also used on reference side. The initial amount of oxygen inside the sensor compartment tube was estimated from the volumetric fraction of oxygen of 21% in the initial air volume in the sensor compartment, where the initial air volume was given by the volume of admixture powder and known total sensor compartment volume. The volume of admixture powder was calculated from the mass of powder and the density of the powder. The densities of the admixture powders Fe–Fe3 O4 and Bi–Bi2 O3 were 3.406 g/cm3 and 12.9 g/cm3 , respectively, according to the measurement based on Archimedes’ principle. For sensor type b and c, the top part of the sensor sealed with liquid adhesive Aron ceramic (containing Al2 O3 ). Sealing with Al2 O3 is usually porous. However, without cracks, the air ingression rate was considered very low compared with rate of chemical reaction in reference electrode in the condition of thick Al2 O3 sealing of 15 mm. In the case of the sensor type a, the volume of gas region inside the sensor tube was minimalized by filling up the gas space with liquid ceramic adhesive Aremco 516 (containing ZrO2 –ZrSiO4 ). The top of the tube was covered with SS304 cylinder and sealed with another liquid adhesive Aron ceramic with the thickness of 15 mm. These two adhesives were inert material and were solidified at high temperature. These two solid sealing materials at the top and inside sensor compartment could reduce the possibility of oxygen ingression from the outside, even if the sealing made with Al2 O3 are usually porous. The galvanic cells in air atmosphere experiment are expressed by Kanthal, Ag, O2 (21%)|MSZ|Fe3 O4 , Kanthal.
2.1. Structure of solid electrolyte sensor Kanthal, Ag, O2 (21%)|MSZ|Bi2 O3 , Kanthal. Fig. 1 shows the schematic drawings of sensors used for the present tests. Solid electrolyte sensors made of magnesia-partially stabilized zirconia with 9% mol of Mg (MSZ, ZR–9 M) from Nikkato Company Japan were used. Both MSZ and yttria stabilized zirconia (YSZ) material as solid electrolyte practically can be used in molten LBE. However, MSZ gave potentials in better agreement with theoretically calculated ones than YSZ [16]. In addition, MSZ is also suitable for molten metal according to the manufacturer. Therefore, we chose the magnesia-stabilized zirconia in the present study. The inner and outer diameters of the sensors were 7 mm and 10 mm, respectively, and the lengths of the sensors were 50 mm for sensors type a and b, and 100 mm for sensor type c as shown in Fig. 1(a). For sensors used in an air atmosphere, a 1 mm-diameter kanthal lead wire as working electrode (WE) was wound on the outer
The cell potential at equilibrium condition E is expressed by Nernst equation:. E(V) =
PO ” RT ln 2 4F PO2 ’
(2)
where F is the Faraday constant, T is the absolute temperature of the sensor, R is the gas constant, PO2 ” and PO2 ’ denotes the oxygen partial pressure in reference side and working electrode side (in air), respectively. The oxygen partial pressure in the working electrode is assumed to be 20,946 Pa only for the experiment in air atmosphere. Fig. 1(b) shows the schematic drawing of the oxygen sensor used in LBE atmosphere. Solid electrolyte sensors made of MSZ were
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Fig. 1. Schematic drawings of sensors, (a) solid metal and metal oxide RE for testing in an air atmosphere, (b) solid metal and metal oxide RE for testing in LBE atmosphere.
Table 1 Binary mixtures for reference electrode (RE). Reference RE type
Powder
Size
Purity (%)
Mass ratio of metal to oxide (−)
Test in air atmosphere
Test in LBE atmosphere
Solid
Fe Fe3 O4 Bi Bi2 O3
325 mesh – 200 mesh <22 mesh
99 95 99.99 99.999
4:1
1.060
0.625
19:1
2.028
1.030
Liquid
used. They were 5 mm in inner diameter, 8 mm in outer diameter and 50 mm in length. Two types of RE were used: Fe–Fe3 O4 and Bi–Bi2 O3 . The total amount of Fe–Fe3 O4 and Bi–Bi2 O3 powders are shown in Table 1. Mo wires with 1 mm of diameter were used as lead wires in RE and WE of LBE. The sensor compartment was filled with liquid ceramic Aremco 516 and sealed at the top by Aron ceramic. The galvanic cells for testing in LBE atmosphere are expressed by
The oxygen partial pressure in oxygen saturated condition in LBE can be expressed [21] by
PO2 ,s
0.5
PO
0.5
2
The oxygen partial pressure in the solid type Fe − Fe3 O4 RE PO2 ”, is calculated by G0 Fe3 O4
(3)
4
The oxygen activity aO in LBE can be expressed by the oxygen concentration in LBE CO and the oxygen partial pressure in LBE PO2 ’ [12] as aO = O CO =
CO = CO,s
PO2 PO2 , s
0.5 (4)
where O is activity coefficient, and the subscript ‘s’ indicates the saturation condition in LBE. Eq. (4) can be re-arranged to Eq. (5).
PO2
0.5
= O CO PO2 ,s
0.5
EX GO,LBE
(6)
RT
= CO exp
EX GO,LBE
(7)
RT
EX The value of GO,LBE relative to 1 wt% standard state is given by [22]
Mo, LBE|MSZ|Bi-Bi2 O3 , Mo
RT ln PO2 ” = 2
= CO,s exp
EX where GO,LBE is excess mollar Gibbs free energy of dissolution of oxygen. The oxygen partial pressure in LBE can be obtained by substituting Eq. (6) for Eq. (5)
Mo, LBE|MSZ|Fe-Fe3 O4 , Mo
Total mass (g)
(5)
EX GO,LBE (J/mol) = -127398+49.272T
(8)
By substituting Eqs. (3) and (7) for Eq. (2), the cell potential at equilibrium condition E is expressed by the oxygen concentration in Fe − Fe3 O4 RE CO as E=
RT 2F
G0 Fe3 O4 4RT
EX GO,LBE
−
RT
− ln CO
,
(9)
As the oxygen partial pressure in Bi–Bi2 O3 RE, PO2 ” is given by G0 Bi2 O3 RT ln PO2 ” = 2 3
(10)
the cell potential at equilibrium condition E is expressed by the oxygen concentration in Bi–Bi2 O3 RE, CO as RT E= 2F
G0 Bi2 O3 3RT
−
EX GO,LBE
RT
− ln CO
,
(11)
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4 Table 2 Types of sensors and experimental conditions. Atmosphere
Reference electrode
Experiment No.
Sensor Type
Final Temperature (◦ C)
Volume of O2 in sensor compartment (mL)a
Mole number of O2 in sensor compartment (mol)a
Air
Fe–Fe3 O4
1 2 3 4 5 6 7
a b c a b d d
600 600 600 600 600 550 550
0.103 0.588 1.400 0.135 0.620 0.044 0.066
4.21 × 10−6 2.40 × 10−5 5.70 × 10−5 5.53 × 10−6 2.53 × 10−5 1.81 × 10−6 2.71 × 10−6
Bi–Bi2 O3 LBE a
Fe–Fe3 O4 Bi–Bi2 O3
Estimated in atmosphere at 25 ◦ C.
In case of oxygen-saturated LBE, CO is equal to the solubility of oxygen CO,S , and expressed by the Schroer correlation [23] as logCO,s (wt%) = 2.2 − 4416/T (T ≤ 1023K)
(12)
2.2. Experimental conditions 2.2.1. Performance in air atmosphere Fig. 2(a) shows the schematic drawing of experimental apparatus used for the test in an air atmosphere with constant oxygen concentra tion of 21%. The sensor was put in the stainless steel vessel. The distance between the sensor tip and vessel surface was 1 cm. The air temperature around the sensor was kept uniform and constant by filling a heat insulator inside a stainless steel vessel. The sensor temperature was measured with the sheathed thermocouple wound on the outer surface of the sensor. Table 2 shows the sensor types and experimental conditions No.1–No.5 in the air atmosphere. Three different types of sensor (a, b, and c) were tested, and the final temperature of this experiment was 600 ◦ C. The cell potential depends on the condition on the reference electrode side. The cell potential was measured using the electrometer Model EM-05 (Toho Technical Research Co., Ltd.) that was specially ordered based on our requirement of specifications for the cell potential measurement, in particular, the input impedance. The input impedance of the electrometer is >1012 G. 2.2.2. Performance in molten LBE atmosphere Fig. 2(b) shows the schematic drawing of experimental apparatus used in the test for the sensors with Fe–Fe3 O4 and Bi–Bi2 O3 REs in molten LBE atmosphere, where the amount of LBE was 450 g. The sensor tube, electrical lead wire, and sheathed thermocouple were inserted in the LBE in the closed cylindrical SS304 crucible within 50 mm in diameter and length. The crucible was evacuated by a vacuum pump and filled with Ar gas with purity 99.99% as a cover gas five times to ensure no air remain inside the crucible. Then, the crucible was heated up, and the cell potential was measured from melting point of LBE to the final temperature of 550◦ C for each sensor. The cell potential was measured using the high impedance electrometer same as previous. Table 2 shows the sensor type and Experiment No.6–No.7 conditions in the LBE atmosphere. 2.3. Control of oxygen potential in molten LBE atmosphere To have a significance difference with the potential in RE side, the oxygen sensor with solid Fe–Fe3 O4 RE was tested in oxygen-saturated LBE and the liquid Bi–Bi2 O3 RE was tested in LBE equilibrium with iron oxide formation. If the sensor with the FeFe3 O4 RE was tested in liquid LBE with Fe3 O4 formation potential, the potential difference of the sensor output signal would be nearly zero voltage. Zero voltage was not good for the observation, since if the signal was shown zero voltage there was the possibility of short circuit. We needed a drastic potential difference between RE
and LBE to see the effect of controlling the oxygen potential by mass-exchanger method clearly. This method to control the oxygen potential is called “mass-exchanger method” that has been studied in static LBE condition [8], and in flow condition in the LBE circulation loop [11]. A numerical model of this method has also been developed [24]. For the test using solid Fe–Fe3 O4 RE, 4 g of PbO powder that was enough according to Eq. (12) was mixed near the surface of the LBE melt to have oxygen saturation condition. Thermodynamic equilibrium is attained through oxygen mass transfer between PbO and the liquid LBE. The chemical reaction of solid PbO, dissolved oxygen [O], and liquid Pb in LBE can be written as PbO(s) ↔ [O]LBE + Pb(l)
(13)
For the test of the oxygen sensor with the liquid metal type RE of Bi–Bi2 O3 , the oxygen potential in LBE was set to be equilibrium with iron oxide formation condition. In order to mix the Fe and Fe3 O4 powder with LBE well even in a static condition, three layers of 150 g LBE at the bottom, mixture of 15 g of Fe powder and 4 g of Fe3 O4 powder layer at the middle, and 300 g of LBE at the top were prepared in the crucible, and melted. Some Fe was necessarily present to be dissolved in LBE and reacted with dissolved oxygen, and the Fe3 O4 was used to keep the equilibrium condition. The reaction of the dissolved iron [Fe], Fe3 O4 and dissolved oxygen [O] in LBE can be written as 3[Fe]LBE + 4[O]LBE ↔ Fe3 O4(s)
(14)
Fe/Fe3 O4 powder should remain as solid phases under the equilibrium condition without dissolving and oxidizing all the Fe into LBE and without reducing all the Fe3 O4 into Fe. It was confirmed that the amount of Fe powder put in LBE was enough according to the Fe solubility limit in LBE [25]: log C Fe,s (wt%) = 2.01 − 4380/T (823 ≤ T ≤ 1053 K)
(15)
The oxygen concentration corresponding to the oxygen potential for the formation of Fe3 O4 in LBE is given by [24]: logCO,min (wt%) = 3.04 −
3 11047 − log CFe,s T 4
(16)
In order to keep this oxygen concentration, Fe3 O4 powder was put enough in LBE taking into account the equilibrium condition in Eqs. (14) and (16). In the case of Fe3 O4 and Fe existing condition, the oxygen potential in LBE is correspondent to that of the F3 O4 formation energy. Therefore, the theoretical cell potential (E) for Bi − Bi2 O3 RE can be written as E=
RT 2F
G0 Bi2 O3 3RT
−
G0 Fe3 O4 4RT
,
(17)
Metallic impurities soluble from the SS304 crucible into LBE might affect the oxygen chemistry in molten LBE. However, the effect of the impurities such as Ni on cell potential was assumed to
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Fig. 2. Schematic drawings of experimental apparatuses for the experiment: (a) air atmosphere and (b) molten LBE atmosphere.
be negligible in this study. From thermochemical-dynamics point of view, nickel may be oxidized in case of controlling using PbO powder, and nickel may be not oxidized in case controlling using Fe/Fe3 O4 powder. In both cases, the amount of PbO or Fe/Fe3 O4 powders was large enough to control the oxygen potential properly. The nickel effect of sensor performance is unknown at present. Therefore, the equilibrium oxygen potential in molten LBE was determined by the presence of PbO or Fe/Fe3 O4 powders.
3. Results and discussion 3.1. Performance of sensors in air atmosphere with constant oxygen potential 3.1.1. Solid Fe − Fe3 O4 reference electrode The working temperature takes an important role in this kind of experiment. According to [1], the threshold temperature is around 360 ◦ C, that is, the temperature that the Nernstian behavior still can be achieved. However, one of the authors found that the threshold temperature was around 450 ◦ C in the same kind of experimental result for solid electrolyte oxygen sensor. The good agreement between measured and theoretical cell potentials was obtained above around 450 ◦ C [26]. Therefore, only the experimental data and theoretical cell potential above the threshold temperature of 450◦ C were included in the later results and discussions. Fig. 3 shows the results of the cell potential in the experiments No 1–No. 3 where the sensors with solid Fe–Fe3 O4 RE was placed in the air atmosphere. The experiment No. 1 that used the sensor type a reached the formation potential of Fe3 O4 faster than the other sensors. The stabilization time of the sensor in experiment No. 1 was the shortest among them. The measured cell potential already agreed with theoretical one after the temperature of the sensor reached 450 ◦ C. The measured cell potentials in the experiments No. 2 and No. 3 were initially higher than the theoretical cell potential at the temperature 450 ◦ C. At 450 ◦ C, the presence of air inside the sensor compartment was still more dominant on cell potential than the formation potential
of Fe3 O4 in experiment No. 2 and No. 3. Therefore, the oxygen partial pressure on the reference side was equal to that of air. The mechanism of the electrode reactions on both surfaces of the solid electrolyte is illustrated in Fig. 4(a). On the solid reference side, the Fe–Fe3 O4 powder just acted like sensing electrode (SE) before the residual oxygen was consumed. Electrochemical reaction of oxygen takes place on the triple-phase boundary (TPB) between Fe metalgas-electrolyte surfaces as an electrode reaction. The reaction is given by: O2(g) + 4e− → 2O2−
(18)
This reaction changes the potential of the SE. The same phenomenon occurs on the working electrode (WE) side, where Ag paste acts as SE When temperature increased, the residual oxygen in the air was consumed gradually by the reaction: 3Fe(s) + 2O2(g) → Fe3 O4(s)
(19)
where the amount of Fe powder inside the sensor compartment was enough for the consumption. As a result, the oxygen partial pressure decreased, and finally the formation and dissociation of Fe3 O4 expressed by Fe3 O4(s) ↔ 3Fe(s) + 2O2(g)
(20)
determined the equilibrium oxygen partial pressure on the reference side The trends of the decrease of measured cell potential in the experiment No. 2 and No. 3 were quite similar to each other after the temperature reached near 600 ◦ C, which suggests that the residual oxygen was consumed in RE in both experiments. The stabilization time at 600 ◦ C of these two sensors was around 10 min. These results gave the evidence that residual air in sensor compartment tube had an effect on the equilibrium condition on the solid Fe–Fe3 O4 RE side. 3.1.2. Liquid Bi–Bi2 O3 reference electrode Fig. 5 shows the results of the cell potential in the experiments No. 4 and No. 5 where the sensors with liquid Bi–Bi2 O3 RE was
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Fig. 3. Cell potential (E) of the sensors in an air atmosphere using solid Fe–Fe3 O4 RE (Experiment No. 1–No. 3). Green, blue, and red colors indicate the Experiment No. 1, 2, and 3, respectively as shown in Table 2. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
placed in the air atmosphere. After the working temperature had reached the threshold temperature of 450 ◦ C, the cell potential (E) of the both of the sensors showed discrepancy with the theoretical E. At 600 ◦ C, the results show that the measured cell potential gradually changed to theoretical value after several hours. However, the stabilization time of the sensor type b in the experiment No. 5 was longer than that of sensor type a in experiment No. 4. The cell potential of the experiment No. 4 during the stabilization time was lower than the theoretical value. The possible explanation for this phenomenon is that the oxygen partial pressure in reference side for experiment No. 4 was initially low since the air was removed by filling with liquid ceramic Aremco 516 (containing ZrO2 –ZrSiO4 ) in sensor compartment. This inert material is used not only to reduce air or gas gap but also to prevent the ingression of air from the outside. Therefore, the cell potential of the experiment No. 4 during the stabilization time was lower than the theoretical value. On the other hand, the experiments No. 2, No. 3, and No. 5 showed higher measured potential than a theoretical one initially because of the presence of air.
The mechanism of the electrode reaction of these two sensors using liquid Bi–Bi2 O3 RE is illustrated in Fig. 4 (b). At RE side, the wettability of electrolyte surface with liquid Bi might poor. Therefore, the gas gap might be formed between electrolyte surface and liquid Bi on the interface, and the reaction can take place either at the double-phase boundary (DPB) of electrolyte and liquid Bi or the triple-phase boundary (TPB) of electrolyte, gas and liquid Bi [27]. When oxygen saturated condition was achieved on the RE side, the O atoms either can be formed from dissociation of O2 molecules or Bi2 O3 . In case of the DPB, the oxygen ions O2− is formed and diffused in the solid electrolyte, and the electrode reaction in liquid RE can be expressed as: 2O ↔ 2O2− + 4e−
(21)
Different behaviors were observed in both of the experiments No. 4 and No. 5, which gave the evidence that air also influenced the cell potential not only for solid type RE but also for liquid type RE through the gas in the gap. The stabilization time of the sensor in the experiment No. 4 is shorter than in the experiment No. 5.
Fig. 4. Electrode reaction models for the oxygen sensors with (a) solid Fe–Fe3 O4 , (b) liquid Bi–Bi2 O3 in air atmosphere.
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Fig. 5. Cell potential (E) of sensors with liquid Bi–Bi2 O3 RE in an air atmosphere (Experiment No. 4–No 5). Blue and red colors indicate the experiments No. 4 and 5 in Table 2, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
It took around 2 h to reach an equilibrium condition for the sensor type a in the experiment No. 4. For the sensor type b in the experiment No. 5, it took around 2.5 h to reach an equilibrium condition. The excess oxygen in the gas region of sensor compartment would remain some time before it diffused and fully consumed by the oxidation of Bi since the liquid Bi was already in oxygen saturated condition. The liquid type RE needed a longer time to reach an equilibrium condition compare to the solid type RE. The oxygen saturated liquid Bi RE could not achieve the equilibrium condition within a short period probably because the effective area of gas phase for the equilibrium reaction of the liquid reference was smaller than that of the solid powder reference. The results of two different volumes for Bi-Bi2 O3 reference electrode in air showed a significant difference of stabilization time. The difference was around 30 min. We could clarify the effect from the two-volume-experiments. On the other hand, for the Fe-Fe3 O4 reference electrode, the difference of stabilization time for each experiment was shorter, i. e., only several minutes. Therefore, it was needed to test at least three different volume for the Fe-Fe3 O4 reference electrode, but it was not for the Bi-Bi2 O3 reference electrode. After the equilibrium condition had been achieved in the liquid RE, the potential of RE was time independent at the constant temperature. In the case of no leakage, the air will not ingress from outside, and the equilibrium condition will remain. The volume of the gas region inside the sensor compartment should be minimalized to have an improvement of design and performance of the sensor. One of the ways is to fill the sensor compartment with inert material as demonstrated above.
3.2. Performance of sensors in molten LBE atmosphere under change in oxygen potential to equilibrium 3.2.1. Solid Fe–Fe3 O4 reference electrode Fig. 6 shows the results of the cell potential in the experiment No. 6 where the sensor with solid Fe –Fe3 O4 RE was placed in molten LBE atmosphere where PbO gradually saturated the oxygen potential. After the LBE had melted in the atmosphere, the achievement to overall equilibrium condition in oxygen sensor system was influenced by the condition of the LBE rather than that condition of RE in the sensor compartment. On RE side, the stabilization time was short, and the equilibrium condition with Fe3 O4 formation potential could be achieved within several minutes as mentioned in
Fig. 6. Cell potential (E) of sensors with solid Fe–Fe3 O4 RE in molten LBE atmosphere where oxygen potential changed to be in equilibrium with PbO formation potential (Experiment No. 6).
Section 3.1.1. Therefore, the dominant factor for the achievement to the overall equilibrium was the change of the condition of working electrode (WE) side of LBE. When the working temperature reached the threshold temperature of 450 ◦ C, the cell potential value was high at first because of low oxygen concentration in LBE, and the cell potential decreased gradually with an increase in oxygen concentration in LBE due to the presence of PbO particle. The mechanism of electrode reaction is illustrated in Fig. 7(a). In LBE, the electrode reaction may take place either at the DPB and TPB. The adsorption-desorption and a partial dissociation of oxygen molecules occur at the DPB. The excess O2 molecules dissolve in LBE and dissociate into O atoms. It was estimated that the PbO powder that dissociated into Pb and O atoms was enough in LBE, and oxygen saturated condition could be achieved. Therefore, the decrease of cell potential observed corresponds to the rise of oxygen concentration in LBE. It was found that the cell potential value was close to PbO formation potential after the temperature reached 470 ◦ C, and finally, the cell potential became constant near the theoretical equilibrium condition. The transient period in LBE from the beginning to reach the equilibrium condition was around 50 min.
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Fig. 7. Electrode reaction models for oxygen sensors with (a) solid Fe–Fe3 O4 , (b) liquid Bi–Bi2 O3 in molten LBE atmosphere.
as oxygen supplier to keep the oxygen concentration in equilibrium with Fe3 O4 formation potential after the amount of oxygen was reduced in the LBE. Slow diffusion of oxygen and Fe in LBE in a static condition caused a relatively long transient period to reach an equilibrium condition. It was reported that the diffusion coefficient of oxygen and Fe in LBE at 550◦ C are 6.3 × 10−6 cm2 /s [21] and 7.6 × 10−6 cm2 /s [28], respectively. In the present result, the total transient period to reached LBE equilibrium with Fe3 O4 formation potential was around 16 h. During this time, the measured cell potential gradually changed until it reached around 0.495 V and became constant at the theoretical equilibrium condition.
Fig. 8. Cell potential (E) of the sensors with liquid Bi–Bi2 O3 RE in molten LBE atmosphere where oxygen potential changed to be in equilibrium with Fe3 O4 formation potential (Experiment No. 7).
3.2.2. Liquid Bi − Bi2 O3 reference electrode Fig. 8 shows the results of the cell potential in the experiment No. 7 where the sensor with liquid Bi–Bi2 O3 RE was placed in molten LBE atmosphere where the oxygen potential gradually changed to equilibrium with Fe3 O4 formation potential. The measured cell potential increased steeply to 0.39 V at first after the working temperature reached 550 ◦ C. However, the equilibrium condition was not attained yet on RE and WE side of LBE. According to the previous result in Section 3.1.2, the stabilization time of RE was around 2 h. Therefore, the dominant factor that influences the sensor output cell potential was the condition of WE side of LBE. The mechanism of electrode reaction is illustrated in Fig. 7(b). It was assumed that the gas gap might also form on the outer surface of the solid electrolyte on the WE side. Therefore, the reaction could take place either at DPB or TPB. The excess oxygen molecules O2 are dissociated in the molten LBE and become oxygen atoms O. The light Fe/Fe3 O4 powder floats on the surface of the LBE due to buoyancy force. The Fe as an impurity diffuses slowly downward in the molten LBE, oxidized by O atoms, and then the product of magnetite Fe3 O4 floats by buoyancy force. At the same time, the oxygen dissolved in the molten LBE diffused upward to the surface and oxidized Fe to form Fe3 O4 . The increase of measured cell potential corresponds to the decrease of oxygen concentration in LBE through these processes. Adding Fe3 O4 powder was required
4. Conclusions The major conclusions of this study are as follows: (1) As the performance of the solid electrolyte oxygen sensor, the effects of an amount of residual air in the reference electrode (RE) on the stabilization time from initial condition to an equilibrium condition was investigated. It was demonstrated that reducing the gas volume in the reference compartment with inert material shortened the stabilization time of reference electrode material to reach an equilibrium condition. (2) The oxygen sensor with the solid Fe–Fe3 O4 reference electrode (RE) was tested in air atmosphere with constant oxygen potential. The cell potential (E) output agreed well with the theoretical one given by Nernst equation after the redox equilibrium condition was achieved in the reference compartment at 600 ◦ C. The stabilization time to the equilibrium in the reference electrode was influenced by the residual air in the sensor compartment. It could be shortened to less than 10 min by decreasing the volume of the gas region in the reference compartment filled with inert material. (3) The oxygen sensor with the liquid Bi–Bi2 O3 RE was tested in air atmosphere with constant oxygen potential. The cell potential agreed well with the theoretical one given by Nernst equation after the redox equilibrium condition was achieved in the reference compartment at 600 ◦ C. The stabilization time to the equilibrium in the reference electrode was influenced by the residual air in the sensor compartment and was longer than that of the sensor with the solid Fe–Fe3 O4 RE even if the volume of residual air was decreased
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by filling the reference compartment with an inert material. The stabilization time was as long as 2 h possibly because the oxygen saturated liquid Bi reference could not achieve the equilibrium condition within a short time. (4) The oxygen sensor with the solid Fe–Fe3 O4 RE that had the shortest stabilization time in the air atmosphere was tested in a static molten LBE atmosphere where the oxygen potential was controlled by PbO. The cell potential agreed with the theoretical one given by the Nernst equation after the oxygen potential in the static LBE reached the equilibrium conditions with the formation potentials of PbO. Since the oxygen potential in the solid Fe–Fe3 O4 RE reached the equilibrium condition faster than in the static LBE. The total transient period to reached LBE equilibrium with PbO formation potential was around 50 min. (5) The oxygen sensor with the liquid Bi–Bi2 O3 RE that had the shortest stabilization time in the air atmosphere was tested in a static molten LBE atmosphere where the oxygen potential was controlled by Fe and Fe3 O4 . The cell potential agreed with the theoretical one given by the Nernst equation after the oxygen potential in the static LBE reached the equilibrium conditions with the formation potentials of Fe3 O4 . Since the oxygen potential in the liquid Bi–Bi2 O3 RE reached the equilibrium condition faster than in the static LBE. The total transient period to reached LBE equilibrium with Fe3 O4 formation potential was around 16 h. References [1] S. Zhuikov, Electrochemistry of Zirconia Gas Sensor, CRC Press, Boca Raton, Florida, 2008. [2] J.W. Fergus, Using chemical sensors to control molten metal processing, J. Miner. Met. Mater. Soc. 52 (2000) http://www.tms.org/pubs/journals/jom/ 0010/fergus/fergus-0010.html. [3] A.M. Azad, S.A. Akbar, S.G. Mhaisalkar, L.D. Birkefeld, K.S. Goto, Solid-State gas sensors: a review, J. Electrochem. Soc. 139 (1992) 3690–3704, http://dx.doi. org/10.1149/1.2069145. [4] J. Zhang, N. Li, Review of the studies on fundamental issues in LBE corrosion, J. Nucl. Mater. 373 (2008) 351–377, http://dx.doi.org/10.1016/j.jnucmat.2007. 06.019. [5] L. Cinotti, C.F. Smith, H. Sekimoto, L. Mansani, M. Reale, J.J. Sienicki, Lead-cooled system design and challenges in the frame of Generation IV International Forum, J. Nucl. Mater. 415 (2011) 245–253, http://dx.doi.org/10. 1016/j.jnucmat.2011.04.042. [6] J. Zhang, Oxygen control technology in applications of liquid lead and lead-bismuth systems for mitigating materials corrosion, J. Appl. Electrochem. 43 (2013) 755–771, http://dx.doi.org/10.1007/s10800-013-0568-8. [7] L. Brissonneau, F. Beauchamp, O. Morier, C. Schroer, J. Konys, A. Kobzova, et al., Oxygen control systems and impurity purification in LBE: learning from DEMETRA project, J. Nucl. Mater. 415 (2011) 348–360, http://dx.doi.org/10. 1016/j.jnucmat.2011.04.040. [8] C. Schroer, J. Konys, A. Verdaguer, J. Abellà, A. Gessi, A. Kobzova, et al., Design and testing of electrochemical oxygen sensors for service in liquid lead alloys, J. Nucl. Mater. 415 (2011) 338–347, http://dx.doi.org/10.1016/j.jnucmat.2011. 04.045. [9] J. Konys, H. Muscher, Z. Voß, O. Wedemeyer, Development of oxygen meters for the use in lead–bismuth, J. Nucl. Mater. 296 (2001) 289–294. [10] J. Konys, H. Muscher, Z. Voß, O. Wedemeyer, Oxygen measurements in stagnant lead–bismuth eutectic using electrochemical sensors, J. Nucl. Mater. 335 (2004) 249–253, http://dx.doi.org/10.1016/j.jnucmat.2004.07.018. [11] M. Kondo, M. Takahashi, Study on control of oxygen concentration in lead–bismuth flow using lead oxide particles, J. Nucl. Mater. 357 (2006) 97–104, http://dx.doi.org/10.1016/j.jnucmat.2006.05.051. [12] G. Muller, G. Schumacher, F. Zimmermann, Investigation on oxygen controlled liquid lead corrosion of surface treated steels, J. Nucl. Mater. 278 (2000) 85–95. [13] A.K. Rivai, T. Kumagai, M. Takahashi, Performance of oxygen sensor in lead-bismuth at high temperature, Prog. Nucl. Energy. 50 (2008) 575–581, http://dx.doi.org/10.1016/j.pnucene.2007.11.043. [14] A.K. Rivai, M. Takahashi, Investigations of a zirconia solid electrolyte oxygen sensor in liquid lead, J. Nucl. Mater. 398 (2010) 160–164, http://dx.doi.org/10. 1016/j.jnucmat.2009.10.027.
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Biographies Pribadi Mumpuni Adhi graduated from Physics Department at Institut Teknologi Bandung (ITB), Indonesia, in 2012 and obtained Master of Eng. in Nuclear Engineering at Tokyo Institute of Technology (Tokyo Tech) in 2015. Currently, he is a Doctoral candidate at Department of Nuclear Engineering, Tokyo Tech. His research focuses on developing oxygen sensor in liquid lead-bismuth eutectic. Masatoshi Kondo obtained his Doctor of Eng. Degree in Nuclear Engineering from Tokyo Tech in 2006. Then, he worked for Nuclear Institute for Fusion Science (NIFS) as an Assistant Professor in 2006. Since 2015, he is an Assistant Professor at Tokyo Institute of Technology (Tokyo Tech). His research focuses on liquid metals and fusion reactor technology. Minoru Takahashi has been a Professor of nuclear engineering at Tokyo Institute of Technology (Tokyo Tech) since 2013. He obtained his Master of Eng. degree and Doctor of Eng. degree in Nuclear Engineering from Tokyo Tech in 1977 and in 1981, respectively. He worked for the development of fast reactors as a researcher at Power Reactor and Nuclear Fuel Development Corporation from 1980 till 1984. Then, he was an Assistant Professor from 1984 till 1997 and an Associate Professor from 1997 till 2013 at Research Laboratory for Nuclear Reactors, Tokyo Tech. His research focuses on thermal-hydraulics and materials for light water reactors, liquid-metalcooled fast reactors, a lead-bismuth-cooled accelerator-driven system and a lithiumcooled fusion reactor blanket.
Please cite this article in press as: P.M. Adhi, et al., Performance of solid electrolyte oxygen sensor with solid and liquid reference electrode for liquid metal, Sens. Actuators B: Chem. (2016), http://dx.doi.org/10.1016/j.snb.2016.10.003